openocd configuration file for `stm32f7` relies on probing to find out
FLASH_ADDR. On nucleo-f767zi board probing (`flash probe 0`) fails when
`srst` is asserted, but `srst` needs to be asserted to be able to flash
the `BOARD` when sleeping or after a hardfault.
To circumvent this in boards/common/stm32/dist/stm32f7.cfg we define a new
flash bank with the appropriate fash start address and specify that this is
the flash bank to be used as default configuration instead of the
default by setting FLASH_BANK=4
